The GWR5600 Class is a class of 0-6-2Tsteam locomotive built between 1924 and 1928. They were designed by C.B Collett for the Great Western Railway (GWR), and were introduced into traffic in 1924. After the 1923 grouping, Swindon inherited a large and variable collection of locomotives from historic Welsh railway companies, which did not fit into their standardisation programme. GWR boiler inspectors arrived en masse and condemned many of the original locomotives. The systematic destruction of many examples of locomotives, most still in serviceable condition, followed. Of the Taff Vale Railway, many engines continued to operate up to the 1950s, but today only two locomotives survived, TVR 'O1' No.28, the last-surviving Welsh-built engine, and TVR 'O2' No.85.

Two hundred GWR 5600 class replacement locomotives were built and remained in service until withdrawn by British Railways between 1962 and 1965. Nine of the class have survived into preservation.

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The railways of South Wales seem to have had a particular liking for the 0-6-2T type. This was because the nature of the work they undertook demanded high adhesive weight, plenty of power with good braking ability, but no need for outright speed, nor large tanks or bunker as the distances from pit to port were short. The 0-6-2 configuration was discovered to confidently handle the sharp curves so prevalent in the area. These Welsh locomotives were taken over by the GWR at The Grouping in 1923 and some were rebuilt with GWR taper boilers. A number of them passed into British Railways (BR) ownership in 1948, including (with some gaps in numbering):

When the GWR took over the Welsh valley lines, they discovered that the Welsh locomotive crews liked their 0-6-2T locomotives. Rather than a new design the 5600 Class was a "Swindonised" version of the Rhymney Railway M class and R class locomotives. The 1904 M class (and the similar 1909 R class) were successful designs ideally suited to hauling heavy coal trains a relatively short distance.

The 5600 Class was specially designed for work in South Wales, replacing the elderly, worn-out locomotives that had been 'inherited' in 1923, when the smaller railway companies were forcibly merged into the GWR at The Grouping. Contrary to this trend, the Rhymney Railway's more modern 0-6-2s were in generally good order and had proved successful. Thus they became the blueprint for the 56xx.

The first of five R class locos was re-boilered by the GWR in 1926 and a single M class was upgraded in 1930. In this form, both were visually almost indistinguishable from the 56 Class.

The design of the 5600 Class followed Great Western Railway practice as far as possible, by utilising many standardised parts. Included in Collett's innovations was a Standard Number 2 boiler which was suitable for the 5600 Class, and the M and R class Rhymney locomotives, complete with the traditional brass GWR safety valve casing and copper-capped chimney.

They were substantial sized tank engines, 37 ft 6ins in length and weighing 62 tons. The side tanks were capable of holding 1900 gallons of water. The high domed cab, bunker and tanks were closely related to the 31xx and 42xx classes. One hundred of the class were built at the GWR workshops in Swindon from 1924-1927.

In 1927 another 100 similar engines were constructed – these were slightly heavier and numbered in the 66xx series. Nos 6600-6649 were Swindon-built in 1927-1928, but due to the pressure of work 6650-6699 were built by Armstrong Whitworth in 1928. This resulted in some minor design differences from the Swindon locomotives.

While they were powerful machines, the 5600s were very unpopular with footplate crews at the time. They were beset by numerous failures, the most common of which was hot axle boxes. They lacked the wider tolerances in their boxes that the original Welsh company locomotives had. They also had the tendency to derail, so those driving them preferred them in reverse, where the pony truck was able to guide them around tight bends. When the Welsh railwaymen discovered that the new GWR 5700 Class 0-6-0 pannier tank (introduced 1929) was even more suitable for the same work – being shorter and lighter, with roughly the same (slightly lower) tractive effort – no further Class 56xx/66xx were built.[2]

The 5600 Class had the distinction of being the only locomotive of 0-6-2 wheel arrangement built new by the GWR. Nevertheless, there were just over 400 of the type in service from 1940–1945, demonstrating the large number acquired in 1923.

A fall in the South Wales coal trade in the 1930s saw many of the class re-allocated to other parts of the system. Due to the stability of the design, many drivers would typically operate the 56xx class down the Welsh valleys in reverse (bunker first). The placement of the trailing wheels helped the engine enter the curves better than if operated in the other direction. Typically, during operation, when pulling a heavy load the tanks were operated bunker first, and then smokebox first on the return trips up the valleys.[4]

All the 56xx/66xx locomotives passed into British Railways ownership at nationalisation in 1948, and all remained in service until 1962, at which time they were withdrawn from service quite rapidly, with the onset of diesel traction on BR gaining momentum. All had been retired by 1965.

As the locomotives were operated mainly in South Wales, some railfans know the Class by the nickname "Taffy Tank";[5][6] 'Taffy' being a derogatory term for someone of Welsh descent. However, the 56xx class never had this title officially, whereas the more typical examples of the Rhymney Railway's M and R classes they replaced, were arguably the original 'Taffy Tanks' of fame.